Transcript PPT
PYTS/ASTR 206 – Exploring the Solar System from the Earth Announcements Homework 1 due… Make sure you give it to Kevin before he leaves Late homeworks can be turned in class on Tuesday February 3rd for 50% credit Kevin has set up a facebook page “Shane Byrne's PTYS 206 class” Can use it to organize homework/study groups etc… 1 Exploring the Solar System from Earth PTYS/ASTR 206 – The Golden Age of Planetary Exploration Shane Byrne – [email protected] PYTS/ASTR 206 – Exploring the Solar System from the Earth In this lecture… Telescopes and how they work Reflectors and refractors Resolution and magnification Atmospheric effects Spacecraft and how they work Fly-bys, Orbiters & Landers Tricks of the trade 3 PYTS/ASTR 206 – Exploring the Solar System from the Earth Why do we use telescopes? 4 PYTS/ASTR 206 – Exploring the Solar System from the Earth 5 Why use telescopes? To make things bigger When light levels are high Very nearby planets Pirate ships Spying on your neighbors Etc… Phoenix lander is a few m across… but a few 100km away! To make things brighter When light levels are low Most of astronomy Far away planets Small objects PYTS/ASTR 206 – Exploring the Solar System from the Earth What have telescopes done for planetary astronomy? Plenty! Heliocentric vs. geocentric solar system Objects visible with the naked eye Sun Moon Mercury (if you’re lucky) Venus Mars Jupiter Saturn Uranus (barely) – still discovered with a telescope Neptune Discovery of Asteroids Discovery of Kuiper Belt Discovery of moons of other planets 6 PYTS/ASTR 206 – Exploring the Solar System from the Earth 7 How do telescopes work? They take light over a wide area and put it into a small area We can do this with either refraction or reflection Bigger is better! Light from distant objects comes in parallel rays The bigger the area of the telescope the more light you can collect The human eye is like an adjustable-size telescope Human eye in daylight •Plenty of light Human eye at night •Iris dilates •Bigger collecting area •You can see fainter things PYTS/ASTR 206 – Exploring the Solar System from the Earth Refractors Light travels slower in glass than air Waves are extended – so they change direction Flat sheets of glass produce no net effect Refraction works in reverse when light leave the glass Light hasn’t been concentrated onto a smaller area 8 PYTS/ASTR 206 – Exploring the Solar System from the Earth Curved pieces of glass (lenses) do produce a change Parallel light converges on a single point – the focal point Distance between the lens and the focal point – the focal length Depends on the curvature of the lens and its size 9 PYTS/ASTR 206 – Exploring the Solar System from the Earth A refracting telescope First lens (objective) gathers light Uses a second lens (eyepiece) to make the light parallel again • So the human eye can use it! Net effect Telescope lens much bigger than eye so more light gathered Things are brighter Magnifies objects Magnification is ratio of focal lengths Magnification = Focal length of objective Focal length of eyepiece 10 PYTS/ASTR 206 – Exploring the Solar System from the Earth The problem with refractors The amount of refraction depends on wavelength Red light and Blue light focus in different places Image gets blurred Chromatic aberration 11 PYTS/ASTR 206 – Exploring the Solar System from the Earth The problem with refractors – cont. We need big lenses to gather a lot of light … but big lenses have long focal lengths Telescopes rapidly get very very unwieldy! 12 PYTS/ASTR 206 – Exploring the Solar System from the Earth Reflectors Pioneered by Isaac Newton Flat mirrors don’t focus light Use curved mirrors to concentrate light These mirrors also have a focal length 13 PYTS/ASTR 206 – Exploring the Solar System from the Earth Why parabolic? Spherical mirrors are easier to make…. Spherical mirrors don’t focus light well Spherical aberration fives you a blurry image Other aberrations can also be corrected Coma Astigmatism Defects in the mirror surface should be small Smaller than the wavelength of light 14 PYTS/ASTR 206 – Exploring the Solar System from the Earth Focus point is in front of the mirror – usable… but unpopular Several designs to get light focused somewhere more convenient Newton’s original design used a flat mirror to redirect focused light to the side If you want to use your eye then you still need an eyepiece Magnification is still just the ratio of the focal lengths 15 PYTS/ASTR 206 – Exploring the Solar System from the Earth Other schemes to redirect the focused light 16 PYTS/ASTR 206 – Exploring the Solar System from the Earth Benefits of reflectors You can make the mirrors huge and the focal length short Keck telescopes – mirrors are 10m across? (built in segments) No chromatic aberration All colors behave the same Plans for a 30m telescope – the CELT California Extremely Large Telescope 3 times the size means 9 times the area! 17 PYTS/ASTR 206 – Exploring the Solar System from the Earth Resolution There’s a limit to what even perfect telescopes can do A single point of light gets spread out a little Called the diffraction limit Resolution – how close can two things be together without joining up. It’s easy to see that there are two separate objects here. 18 PYTS/ASTR 206 – Exploring the Solar System from the Earth As things get closer together we can no longer the individual objects The closest angularseparation they can have and still be separate is the resolution. Depends on size of telescope Depends on wavelength of light Same principle to know what the smallest feature on a planet you can see is. 19 PYTS/ASTR 206 – Exploring the Solar System from the Earth Example Smallest object/separation that we can resolve = Warning: This formula produces radians wavelength Size of telescope The textbook has a formula that produces arcseconds… Can we see the Apollo lander on the Moon? The lander is almost 4m across The Moon is 384,000,000m away Angular size 10-8 radians Wavelength of visible light 5*10-7 m Size of Keck telescope is 10m Resolution of Keck 5*10-8 radians 20 PYTS/ASTR 206 – Exploring the Solar System from the Earth Can we see the Apollo lander on the Moon? The lander is almost 4m across The Moon is 384,000,000m away Angular size 1*10-8 radians Wavelength of visible light 5*10-7 m Size of Keck telescope is 10m Resolution of Keck 5*10-8 radians We can’t resolve the lander. We’d need a telescope 50m across to be able to see the Apollo lander. 21 PYTS/ASTR 206 – Exploring the Solar System from the Earth Increasing Magnification But the Same Resolution Same Magnification Increasing Resolution 22 PYTS/ASTR 206 – Exploring the Solar System from the Earth 23 Atmospheric effects The diffraction is only a theoretical ‘best-case scenario’ Earth’s atmosphere is a pretty turbulent place Especially the lower atmosphere – observatories are on mountains! Makes stars twinkle Astronomers call this effect ‘seeing’ Lousy seeing =Lousy images Good Seeing =Good images Typically, seeing ~ 0.5 arcsec (~2.5*10-6 radians) E.g. on the Moon, that’s a feature ~1km across Equivalent to a telescope only 10cm in diameter !!! PYTS/ASTR 206 – Exploring the Solar System from the Earth 24 What’s the problem? Neptune Different parcels of air have different temperatures Light-wave gets bent and warped Telescope PYTS/ASTR 206 – Exploring the Solar System from the Earth What’s the solution? Adaptive optics - Flexible mirror Flexible mirror Mirror deforms in a way that cancels out the Atmosphere changes all the time Mirror updates its shape many time per second 25 PYTS/ASTR 206 – Exploring the Solar System from the Earth How does the mirror know what to do? You need a nearby guide-star Starlight passes through the same patch of atmosphere as planets light Star is supposed to be a point Wavefront sensor detects distortion… …and figures out how to warp the mirror Usually there’s no natural guide star So we use a laser Reflects of a specific layer high in the atmosphere High sodium layer from meteorite burn ups 90-100km high 26 PYTS/ASTR 206 – Exploring the Solar System from the Earth Atmospheric effects – cont. We can’t use all wavelengths from ground-based telescopes Gases in our atmosphere absorb light at many wavelengths This time there’s no real way around the problem Atmosphere screens out Some infrared wavelengths Some microwave frequencies Most UV light – Good! X-rays – Very Good! Gamma Rays – Very very Good!! 27 PYTS/ASTR 206 – Exploring the Solar System from the Earth By-passing the Atmosphere is the best option… Hubble and its successor Infrared - Spitzer 28 PYTS/ASTR 206 – Exploring the Solar System from the Earth Spacecraft Fly-bys Usually once off encounters Can swing by multiple planets (Voyager) …rarely the same planet multiple times Orbiters Usually just one destination (can’t carry the fuel needed to escape) Long-term monitoring – missions can last years Landers Touch-down on solid planets Parachute into gas giant planets Different type of instrument Lots of hybrids Lander/Flyby – Deep Impact Orbiter/Flyby - Cassini 29 PYTS/ASTR 206 – Exploring the Solar System from the Earth Gravity assist 1st tried by mariner 10 Common now for missions Momentum transfer with a planet Big effect on spacecraft velocity Tiny effect on planet’s velocity Narrows your range of launch dates 30 PYTS/ASTR 206 – Exploring the Solar System from the Earth Launch windows Earth moves at 27 km s-1 We don’t want to waste that energy To get to Mars - Earth is in a favorable position every two years 31 PYTS/ASTR 206 – Exploring the Solar System from the Earth In this lecture… Telescopes and how they work Reflectors and refractors Resolution and magnification Atmospheric effects Spacecraft and how they work Fly-bys, Orbiters & Landers Tricks of the trade Next: Exploring the solar system from the Earth Reading Chapter 6 to revise this lecture Chapter 16 for next Tuesday 32